When preparing wood fibres a separation of fibres from steam takes place in a fibre separation step where a cyclone or a centrifugal separator is used for this purpose. Preferably the steam is recycled also from this step. The pressure in the system before the fibre separation is in most cases 3-12 bar. The following process step may be the drying of the fibre. This takes place at a lower pressure, sometimes as low as atmospheric pressure. In order to prevent the steam from escaping to the drying step at lower pressure a pressure tight feeding of the fibre from the separation step is required.
When treating mechanical pulp for paper manufacturing this can be easily solved by a plug screw where the fed fibres are compressed and form a steam tight plug in the plug screw at the same time as it is further fed and subsequently may be fed to further treatment at a lower pressure.
On the contrary, when treating mechanical pulp for manufacturing of, for example, fibre board or MDF-technology, it is not possible to compress the fibres since lumps are thus formed, which cause problems at the drying of the fibres and at following manufacturing of end products. Instead a sluice feed may be used in which the separated fibre fall down into a pocket in its rotor provided in the sluice feeder, which rotor is rotated and the fibre falls out in the lower portion of the sluice feeder where a lower pressure may prevail without being compressed. The rotor and its pockets are sealed against the periphery of the sluice feeder.
A problem with traditional sluice feeders is that they cannot be driven at high speed since the fibres then will be thrown out of the rotor and thus the capacity will decrease. Since these sluice feeders must be rotated slowly they must be made large and voluminous in order to be able to sluice desired quantity of fibre in a certain period of time so that the production is not obstructed.